This invention relates to a technique for reducing the harmonic currents in alternating-current (AC) distribution networks through the use of polyphase Zig-Zag autotransformers ("Zig-Zag transformers") with a choke (inductor) installed in the neutral line.
A common problem in power distribution networks includes AC line currents having high harmonic content and hence the potential for overloaded neutral wires caused by the non-linear loading of electronic power supplies. Different solutions have been offered for this problem. Some examples include specially designed K-rated AC distribution transformers, delta connected primary windings, and L-C (inductor-capacitor) tuned filters.
All of the above methods have some limitations. For instance, a K-rated transformer does not eliminate harmonics, and may transmit them into the feeder. Neutral currents that flow from various loads to the K-rated transformer might still be very high. Often, these K-rated transformers are more expensive and are larger in physical size than conventional transformers.
The delta connected primary of a power distribution transformer can only eliminate triplen harmonics (zero-sequence currents) for balanced loads. Additionally, neutral currents caused by the loads are usually not eliminated. The primary side circuit breaker might also not protect a transformer against overcurrents because the circuit breaker may not sense the triplen harmonic current that is circulating in the primary of the transformer.
L-C filters can often create undesirable resonances, which can lead to an increase in harmonic currents, thus intensifying the overall problem.
Another solution includes the use of Zig-Zag transformers installed in the distribution network. Zig-Zag transformers have previously been used to reduce harmonics and eliminate neutral currents by transferring harmonic currents between the phases of a three-phase system. See Robert H. Lee, Eliminating Harmonic Currents Using Transformers, Power Quality, September/October 1991, at 33.
However, large neutral currents often return back into the distribution network and are not properly eliminated by the Zig-Zag transformer due to the low source neutral impedance as compared to the relatively high Zig-Zag transformer impedance. These large returning neutral currents can impede the ability of the Zig-Zag transformer to perform, for example, the following functions: (1) the localization of the neutral currents, (2) the elimination of triplen harmonics (zero-sequence currents) from the phase currents, and (3) the balancing of the phase currents. This can additionally lead to overheating of distribution transformers and secondary distribution wires.
It is therefore an object of the present invention to reduce the harmonic content of AC line currents in power distribution networks through the use of Zig-Zag transformers with a strategically-placed choke installed in the neutral line.